(1) Field of the Invention
The present invention relates to an improvement in a metal electrode for continuously measuring changes in the oxygen partial pressure of a living body. More particularly, it relates to an improvement in a metal electrode for measuring the oxygen partial pressure by utilizing the principle of the polarography, in which the measurement precision and stability are increased.
(2) Description of the Related Art
Methods for electrically measuring concentrations of living body components in blood or tissues by using electrodes have long been known. In particular, methods utilizing the principle of polarography have been widely used for measuring concentrations of the oxygen gas component and various ion components, especially for continuously measuring changes in the concentrations of these components. Although various components are measured as living body components in blood or tissues, the following description will be made with reference to the measurement of the oxygen partial pressure as an example. In the measurement method utilizing polarography, an electrode of a noble metal such as gold, platinum or silver and a reference electrode of silver/silver chloride or the like are used, and a micro-voltage is imposed between both electrodes to effect a reduction of oxygen on the surface of the working electrode (cathode). The oxygen concentration in a solution is determined by measuring the reduction current generated.
The oxygen gas concentration (oxygen partial pressure) in a living body has an important influence on that body. For example, it is considered important to know, precisely and continuously, any changes in the oxygen partial pressure in new-born babies or during anesthesia, cardiac surgery, brain surgery, and digestive organ surgery. There is also an increased demand to measure changes in the oxygen partial pressure by inserting the electrode directly into the blood vessel or living body tissue, when such measurement is considered necessary.
An important factor in the above-mentioned measurement method is the diffusion current based on the oxygen concentration gradient between the surface of the cathode and the solution. Movement of the cardiac muscle and pulsation of the blood is constant and continuous in a living body, and the diffusion current is greatly influenced by these motions of the living body. Therefore, it is very difficult to precisely measure a small oxygen partial pressure. ExtensiVe efforts have been made heretofore to eliminate this defect. Namely, there has been proposed a composite electrode, comprising working and reference electrodes and an electrolyte, which are enveloped in an oxygen-permeable membrane (see U.S. Pat. No. 3,957,613). Also, there has been proposed a method in which the surface of a working electrode is covered with a hydrophilic water-swelling membrane of polyhydroxyethyl acrylate, cellophane or the like, so that oxygen is moved to the surface of the electrode through water captured among the polymer molecules (see U.S. Pat. No. 3,912,614). These composite electrode and method have been practically adopted. However, the composite electrode has a large size and, therefore, it can be inserted only into a specific portion, for example, a large blood vessel. In the above-mentioned method, the measurement sensitivity varies depending upon the particular state in which the water-swelling membrane is held and, hence, the measurement precision is low. Furthermore, the membrane becomes brittle and is readily broken on drying. We carried out research into the development of an electrode for a living body which can be inserted into any portion of the tissue and blood vessel of the living body and is also capable of precisely measuring the oxygen partial pressure continuously and stably without being influenced by motion of the tissue or blood vessels. Consequently, in Japanese Unexamined Patent Publication No. 57-117838, we proposed an electrode for a living body, which comprises a metal wire electrode covered with a porous membrane. However, this membrane-covered electrode is suffers from insufficient adhesion between the insulating covering layer and the metal wire and the adhesion of the porous membrane, and thus the measurement cannot be performed stably for a long period. Moreover, when many electrodes are produced under the same conditions, large deviations in the output values are observed in the formed electrodes.
Accordingly, development of an electrode for a living body, in which the foregoing defects are eliminated, has been eagerly desired.